(1. ) Field of the Invention
The present invention relates generally to circuits for processing chrominance signals in a color video reproducing apparatus, and more particularly is directed to a circuit for minimizing cross-talk contained in a chrominance signal separated from a color video signal reproduced from a magnetic tape on which the color video signal has been recorded in a plurality of successive oblique tracks.
(2. ) Description of the Prior Art
In a widely used video tape recorder (VTR), when recording a color video signal on a magnetic tape, a frequency-modulated luminance signal (hereinafter referred to as an FM luminance signal), which is produced by frequency-modulating an appropriate carrier by a luminance signal separated from the color video signal and a chrominance signal also separated from the color video signal and frequency-converted to have a frequency band lower than the frequency band of the FM luminance signal are mixed or combined to form a composite video signal which is recorded in a plurality of parallel oblique tracks on the magnetic tape. The oblique tracks are formed by a pair of rotary magnetic heads which are supplied with the composite video signal and driven to alternately scan the magnetic tape obliquely in respect to the running direction of the magnetic tape.
For such recording of the color video signal, two operational modes are selectively used in some video tape recorders. In one of these operational modes, which will be referred to as an SP or short-play recording mode and which is illustrated in FIG. 1A of the accompanying drawings, the speed at which the magnetic tape is transported is made to be relatively high so that the pitch P.sub.1 between each two adjacent oblique tracks T is larger than the width W of each of the rotary magnetic heads and, therefore, the oblique tracks T are formed on the magnetic tape with spaces or so-called guard bands g between adjacent tracks. In the other mode, which will be referred to as an LP or long-play recording mode and which is illustrated in FIG. 1B of the accompanying drawings, the speed at which the magnetic tape is transported is limited or reduced so that the pitch P.sub.2 between each two adjacent oblique tracks T is smaller than the width W of each of the rotary magnetic heads and, therefore, the oblique tracks T are formed immediately adjacent each other on the tape without guard bands between the adjacent tracks so that the recording density of the composite video signal on the magnetic tape is increased and thereby the duration of the recording can be increased.
For reproducing a composite video signal recorded on the magnetic tape using the SP or LP recording mode, a reproducing mode is employed in which the oblique tracks arranged on the magnetic tape are alternately scanned in succession by a pair of rotary magnetic heads which reproduce the FM luminance signal and frequency-converted chrominance signal from each of the oblique tracks and a reproduced color video signal is obtained based on the outputs of the rotary magnetic heads. In such case, the problem of "cross-talk" between closely arranged oblique tracks arises in reproduction of the FM luminance signal and the frequency-converted chrominance signal. For the purpose of suppressing such cross-talk, the rotary magnetic heads used for recording the composite color video signal on the magnetic tape are provided with gaps having different azimuth angles, so that each adjacent two of the oblique tracks are recorded by rotary magnetic heads with different gap angles (head azimuth angles), respectively, for example, as are shown for the oblique tracks T in FIGS. 1A and 1B. Subsequently, during reproduction, each oblique track is scanned by the rotary magnetic head Ha or Hb (FIGS. 1A and 1B) having the corresponding gap angle for reading the composite color video signal therefrom, with the result that a beneficial azimuth loss is experienced in respect to the cross-talk derived from adjacent oblique tracks.
With such arrangements of the gap angles of the rotary magnetic heads, substantial azimuth loss and corresponding reduction in cross-talk is obtained in respect of the FM luminance signal which resides in a relatively high frequency band. Therefore, the cross-talk in respect to the FM luminance signal reproduced from the magnetic tape by the rotary magnetic heads is sufficiently diminished. However, the azimuth loss is not very effective for cross-talk in respect of the frequency-converted chrominance signal which resides in a relatively low frequency band, so that other measures are taken for minimizing cross-talk in respect of the frequency-converted chrominance signal read from the magnetic tape by the rotary magnetic heads. For example, the cross-talk in respect of the frequency-converted chrominance signal reproduced by the rotary magnetic heads from the magnetic tape is substantially eliminated by recording the frequency-converted chrominance signal on the magnetic tape with its carrier having constant phase in alternate oblique tracks and with its carrier reversed in phase at every horizontal period in the other alternate oblique tracks, and by frequency-converting the frequency-converted chrominance signal reproduced from the magnetic tape so as to provide the carrier thereof with its original frequency and a predetermined phase and then making it pass through a so-called comb-filter.
FIG. 2 shows a basic or simple comb-filter previously proposed to be used for suppressing the cross-talk in the reproduced chrominance signal as mentioned above. This simple comb-filter is shown to have an input terminal 1 from which the reproduced chrominance signal is supplied directly to one input of a subtracter 3. The reproduced chrominance signal is also supplied through a delay device 2 providing a delay of one horizontal period to another input of the subtracter 3, and an output of subtracter 3 is lead to an output terminal 4.
When using the comb-filter shown in FIG. 2, it is possible that the cross-talk will not be sufficiently diminished if the reproduced chrominance signal contains a large amount of cross-talk in respect of the frequency converted chrominance signal. Accordingly, a feedback-type comb-filter has been also proposed to be used for suppressing the cross-talk in the reproduced chrominance signal, for example, as specifically disclosed in Japanese patent application published before examination under publication No. 56/60186. In such a feedback-type comb-filter, as shown in FIG. 3, the reproduced chrominance signal from an input terminal 1 is supplied to one input of a subtracter 5 and an output of subtracter 5 is supplied directly to one input of a subtracter 3 and, through a delay device 2 which delays the output of subtracter 5 by one horizontal period, to the other input of subtracter 3. Once again, the output of subtracter 3 is connected to an output terminal 4. Further, the output of subtracter 5 is also supplied directly to one input of an adder 6 which has another input connected to the output of delay device 2 and an output of adder 6 is fed through a feedback level controller 7 to another input of subtracter 5 so as to form a feedback loop.
With the feedback-type comb-filter shown in FIG. 3, the reproduced chrominance signal supplied to input terminal 1 is subjected repeatedly to substantial cross-talk suppression by the simple comb-filter which is constituted by delay device 2 and subtracter 3. Therefore, cross-talk in respect of the reproduced chrominance signal is sufficiently diminished or minimized, so that the chrominance signal appearing at output terminal 4 is processed to have an improved signal to noise (S/N) ratio.
As described above, when the feedback-type comb-filter is used for suppressing cross-talk in respect of the reproduced chrominance signal, it is expected that the cross-talk will be sufficiently diminished or minimized even though the reproduced chrominance signal contains a large amount of cross-talk. In such case, however, since the reproduced chrominance signal passes repeatedly through the simple comb-filter for providing the chrominance signal with an improved signal to noise (S/N) ratio at the output terminal 4, there is the problem that the chrominance signal appearing at output terminal 4 causes so-called color penetration in the vertical direction on a picture obtained from a reproduced color video signal containing the chrominance signal appearing at output terminal 4. This problem will be hereinafter referred to merely as the problem of vertical color penetration.
Returning to the discussion of the recording of the color video signal on the magnetic tape, it will be seen that the width w.sub.1 of each oblique track on the magnetic tape in which the composite color video signal is recorded in the SP recording mode, such as the oblique track T shown in FIG. 1A, is wider than the width w.sub.2 of each oblique track formed on the magnetic tape when using the LP recording mode. Further, as earlier noted, each oblique track recorded on the magnetic tape when using the SP recording mode is accompanied by the guard bands at the opposite sides thereof. As a result, there is a difference between the reproduced chrominance signal obtained in a first reproducing mode, which will be referred to as an SP reproducing mode, and in which oblique tracks recorded on the magnetic tape by using the SP recording mode are scanned alternately by two rotary magnetic heads Ha and Hb each having a width W smaller than the pitch P.sub.1 between each two adjacent oblique tracks which are separated by guard bands g, as shown in FIG. 1A, and the reproduced chrominance signal obtained in a second reproducing mode, which will be referred to as an LP reproducing mode, and in which oblique tracks recorded on the magnetic tape by using the LP recording mode are scanned alternately by the two rotary magnetic heads Ha and Hb each having a width W larger than the pitch P.sub.2 between each two adjacent oblique tracks recorded without the guard bands, as in FIG. 1B. In other words, the reproduced chrominance signal obtained in the SP reproducing mode contains a relatively small amount of cross-talk so that its S/N ratio is not greatly reduced, while the reproduced chrominance signal obtained in the LP reproducing mode contains a relatively large amount of cross-talk by which its S/N ratio is considerably deteriorated.
Accordingly, the simple or non-feedback type comb-filter shown in FIG. 2 may be used for sufficiently suppressing the cross-talk contained in the reproduced chrominance signal so as to obtain a chrominance signal of high quality only when the SP reproducing mode is being employed. However, the cross-talk suppression by the simple or nonfeedback-type comb-filter is not sufficient when the reproduced chrominance signal is obtained in the LP reproducing mode and therefore contains a large amount of cross-talk. On the other hand, when the feedback-type comb-filter shown in FIG. 3 is used for suppressing the cross-talk contained in the reproduced chrominance signal obtained in the LP reproducing mode, a chrominance signal processed to sufficiently eliminate or minimize the cross-talk and to have high quality is obtained at the output terminal of the feedback-type comb-filter. However, a chrominance signal having an excessively improved S/N ratio and giving rise to problems, such as, vertical color penetration as mentioned above, appears at the output terminal of the feedback-type comb-filter if the latter is used in the SP reproducing mode.